They eat feed intended for cattle. Ranchers who have had me as guests have asked me to shoot them and I did, and enjoyed myself. They are absolutely pests, and their existence can be at a cost to humans. It's no different than killing mice in your home, except this can be fun. There is nothing unethical about having fun doing something that needs done anyway.
It would be more unethical not to kill them when you have the equipment, ability, and time to do so. It's like when the men of 2nd Ranger Battalion came upon the machine gun nest. They could have gone around it and still accomplished their mission, but ethically it would have been wrong to leave it there to ambush the next group of soldiers that came through.